$\bullet$ During respiration, process of ATP synthesis is explained by chemiosmotic model. it says that a proton gradient is required for ATP synthesis that is established by oxidation-reduction reactions.

$\bullet$ In ETC, one NADH + H⁺ produces 3 ATP while one FADH₂ produces 2 ATP molecules.
NADH and FADH₂ are two different types of electron donors. They differ in the ways they feed electron during electron transport chain. NADH feeds its electrons into the electron transport chain at the beginning (Complex I). FADH₂ feeds into the electron transport chain at Complex II (at a lower energy level down the chain). The high energy electrons from NADH have sufficient energy to result in 3 ATP whereas the lower energy electrons in FADH₂ have energy for 2 ATP production.

$\bullet$ ATP is synthesized via complex V.

$\bullet$ In ETS, oxygen acts as terminal electron acceptor.